The invention relates to a drill bit of the twist drill type with turns which are held together by a drill centre web and each issue in a radially extending cutting edge of which one extends to the bit axis and the other is shorter than it.
A drill bit of this type is described in DE-OS 30 20 948 (Hertel). The design of the known drill bit has the result that the drill bit may also be produced with smaller diameters.
Drill bits of the type mentioned at the outset may be used for both drilling and counterboring, blind holes which end with a flat base being predominantly produced in the latter case.
The drilling of holes and the subsequent counterboring of an ensuing blind hole represents a procedure which has proven particularly advantageous for the arc welding of aluminium sheets. The welding of two aluminium sheets prepared in this way involves a respective weld-in part made of aluminium for each hole and blind hole which is caused to melt on the base of the blind hole by arc heating and thus joins the two sheets together by means of the molten metal. A process of this type is described, for example, in our U.S. application Ser. No. 07/923,393.
The invention relates to the drilling of a hole and the subsequent counterboring of an ensuing blind hole in two superimposed aluminium sheets using a drill bit such that arc welding can be carried out directly after the drilling and counterboring process without the need for work on the aluminium sheets in preparation for welding.